1. Field of the Invention
The present invention relates to a digital fault locator for determining a distance from the locator to a point at which a fault of a power transmission line occurs.
2. Description of Prior Art
As a fault locator for a power transmission system, a surge receiving system in which a surge produced at a fault point is received at both terminals of the transmission line and the fault point is located by a difference in receiving time between the terminals and a pulse radar system in which a pulse is transmitted through a power transmission line in which a fault occurs after the fault is detected and a reflection is measured have been used practically. In these systems, however, a signal transmission device is required in parallel to the power transmission line and a blocking coil for blocking a pulse leakage is required, respectively, resulting in an increased cost therefor.
On the other hand, due to the recent development of microcomputers, various fault locating systems have been proposed in which a distance to a fault point is calculated by using voltage and current data of the transmission system to thereby locate the fault point economically.
In a usual digital fault locator using a microcomputer, voltage/current digital data obtained in the transmission line having the fault are stored in a memory circuit and a fault location is performed by a calculation using these data according to known equations.
There are many equations for use in locating the fault point, and, for example, the following equation has been proposed (IEEE, PAS-101, No. 8, 1982, 82WM 088-3). ##EQU1## where X is a distance to a fault point, V.sub.s is a voltage at a point at which a locator is provided, I.sub.s is a current measured at the point of the locator, I.sub.s " is a difference between currents measured at the points before and after the fault occurs, Z is an impedance of a unit length of the power transmission line, I.sub.m is an imaginary part and * represents a conjugate complex number.
It is usual that voltage and current of the transmission line in which the fault occurs may contain high harmonics. Particularly, this tendency is increased by recent increased use of facilities such as phase regulation equipments and underground cables. Therefore, in calculating the fault point by using the equation (1), such high harmonics contained in the voltage/current detected in the transmission line are removed by passing them through a low pass filter to obtain the fundamental component.
An analog filter is disposed in a preceding stage to an A/D conversion of an a.c. electric input amount in a usual digital device to remove higher harmonic components. Therefore, the filtering should be performed, after the A/D conversion, in a digital manner by using a microcomputer.
The fault locator is adapted to measure the distance to the fault point accurately and, therefore, an attenuation effect of such digital filter should be as high as possible.
On the other hand, a response of such filter having high attenuation effect is usually slow and thus it is necessary to have a longer time in obtaining a correct calculation.
It is usual that, so long as a protection device operates normally, the fault phenomenon of a power transmission system disappears within a time range from 50 ms to 80 ms. In view of this, the use of a filter whose response is too low is not practical. The time for which the fault of the system persists depends upon the phase of system voltage when the fault occurs and a response time of a circuit breaker provided in the system, etc. Therefore, the filter function should be determined by taking these matters into consideration.
In order to overcome these problems, a digital fault locator has been proposed in Japanese Patent Application Laid-Open No. 204219/1985, which comprises a memory for storing digital data of voltage and current of a power transmission system and a plurality of digital filters having different filter functions, to which the digital data stored in the memory are supplied. Outputs of these digital filters are selectively used according to the time from a detection of the fault to measure the distance to the fault point in real time.
In the fault locator of this type in which the filter outputs are selectively used in real time and a locating operation is performed for each filter output, the time given to the operation is usually too short and, thus, it is sometimes difficult to obtain a desired result of operation. Therefore, the proposed locator is not so practical.